Extreme checklist to look for odd life on other worlds

When deciding which planets beyond Earth could host life, astronomers usually follow the water. Exoplanets with rocky surfaces are declared habitable if they orbit far enough from their star to be warm, while potentially supporting oceans and seas.

But as our planetary collection grows, and our telescopes for studying them improve, some astrobiologists say it is time to narrow the search. Taking what we know about the extremes that life can endure, Christopher McKay at NASA's Ames Research Center in California has come up with an expanded checklist for habitability.

"If we go through that checklist and, bang-bang-bang-bang, we've got it all, that is incredibly exciting," says McKay. "Then we have a compelling case for a planet with life." The list may one day pinpoint not only benign, Earth-like environments, but also worlds that may host other forms of life in conditions that would kill humans outright.

Some items on the checklist can be inferred just from knowing a planet's size, mass and distance from its host star. Others will require directly photographing the planets and probing the contents of their atmospheres. Spacecraft with those capabilities, like the proposed Starshade mission and the upcoming James Webb Space Telescope, are already being developed.

"That doesn't mean we are positioned to do it yet, but the discussion has gotten very detailed and very specific and perhaps more real," says Adam Burrows at Princeton University. "In the next few decades we may have a plan for determining whether life exists elsewhere."

When the right instruments come online, here are some of the wildest things we may find thriving on exoplanets, according to McKay's checklist:

1. Frozen or boiling bugsThe habitable zone around a star is largely based on where an orbiting planet would have a relatively mild temperature range, so water can be a warm, pleasant liquid. But McKay points to microbes on Earth that can survive well below freezing or above boiling temperatures. In 2013, researchers at McGill University in Montreal, Canada, found microorganisms in Arctic permafrost that were able to reproduce at -15 °C. And researchers have grown microbes in the lab at temperatures as high as 122 °C. An icy world could be covered in something akin to Chlamydomonas nivalis, or watermelon snow – a red-coloured alga that only grows in freezing water.

2. Arid algaeIt is also possible for a planet to host life even if it has barely any water, says McKay. Cyanobacteria live under and inside rocks in the Atacama Desert in Chile, which gets only a few days of rain or fog every year. And lichens grow in the Antarctic Dry Valleys, only occasionally getting small sips of water from melting snow. "You don't need a Pacific Ocean," says McKay. "A planet like the fictional world in Dune would be habitable, though you may not have sand worms."

3. Deep, dark seaweedAny life on another world would need sufficient starlight or geothermal energy sources to drive its vital processes. Luckily you don't need a lot of sunlight to drive photosynthesis – sea plants called red macroalgae can grow in deep water with just one per cent of the sunlight we receive on Earth's surface.

4. Radiation-proof bacteriaAnyone who has been sunburned knows the dangers of too much ultraviolet radiation. Humans and other complex organisms are sensitive to UV light and other radiation, like the cosmic rays that stream down from space. But microbes are much hardier. The most extreme example is Deinococcus radiodurans, the world's toughest bacterium, which can survive in the sort of conditions you might find inside a nuclear reactor.

5. Oxygen haters, nitrogen loversIt is generally agreed that abundant oxygen or ozone in the atmosphere would be a sure sign that complex life already exists on a planet, although it's not the only one. But some bacteria, like the Actinomyces genus found in soils and compost, cannot function when there's oxygen around. Other bacteria don't use oxygen for growth but will tolerate its presence. Instead of lots of oxygen, we may first need to find worlds rich in nitrogen. Nothing we know of can survive without nitrogen, which is essential for building proteins and DNA. "It's hard to imagine a kind of life that isn't going to need lots of nitrogen," says McKay.

However, there is one place in our solar system that could change this list entirely. Saturn's moon Titan has an atmosphere, liquids on the surface and even a weather cycle. But instead of water, its liquids are methane and ethane, and its atmosphere is a choking haze of nitrogen and methane. But Titan has also shown evidence that it has complex molecules that may be building blocks for life. "Titan is a little reminder that there are perhaps more things in heaven and Earth than we can imagine, as Hamlet said. It's a cautionary tale," says McKay. "If we discover something new, we will have to rewrite this chapter."

Journal reference: PNAS, DOI: 10.1073/pnas.1304212111

Correction, 23 June 2014:When this article was first published, it described Chlamydomonas nivalis as a red alga. It is in fact a green alga with a red pigment.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.